Disk Space

So I purchased a new laptop, now I am wondering how to use the other disk storage on my computer.

You can either just create a folder and move the stuff you want into it on that drive or when installing a program just change the letter to the drive desired.

Is there a way to say for so merging the space? Like adding the space from Disk D and E to my OS C?

Almost positive you could only do that with a raid arrangement. They have to be the same HDD or SSD for that to work to my knowledge though.